Guest “geology lessons” by David Middleton
One of the things I love about writing for Watts Up With That, is the fact that reader comments often inspire me to research and write subsequent posts. In my recent post about the origins of the Moon, one commentator suggested that the rate of lunar recession (tidal acceleration) indicated that the Earth was much younger than 4.5 billion years old and/or somehow disproved the geological Principle of Uniformitarianism. I didn’t give much thought to my reply. I simply calculated the distance from the Earth to the Moon 1 billion and 4.5 billion years ago. The Moon is currently receding (moving away) from the Earth at a rate of about 3.8 cm/yr. This has been directly measured with lasers.
At 3.8 cm/yr, the Moon would have been 215,288 miles away from Earth a billion years ago. It is currently an average of 238,900 miles away. At 3.8 cm/yr, it still would have been 132,646 miles away 4.5 BY.
If the Moon did did originate from a collision with Earth, it would have been a lost closer to Earth 4.5 BY than 100,000 miles.
At first thought, 215,288 miles apart didn’t seem problematic. It is well outside the Earth-Moon Roche Radius, however, at a steady rate of 3.8 cm/yr, the Moon’s orbit would have been close enough to Earth, that 1.5 billion years ago, the tidal forces of the Earth-Moon system may yielded catastrophic results:
Tracing the history of Earth’s tidal deceleration and the evolution of the Moon’s orbit is a major challenge for geology. The implications of employing the present rate of tidal energy dissipation on a geological timescale are catastrophic: Around 1500 Ma the Moon would have been close to Earth, with the consequence that the much larger tidal forces would have disrupted the Moon or caused the total melting of Earth’s mantle and of the Moon [Lambeck, 1980].
Apart from the classic bad science fiction movie, 2012, or the junk science of Hapgood and Velikovsky, there is no evidence whatsoever, of lunar disruption or the total melting of Earth’s mantle 1.5 billion years ago.
So… How do we solve this conundrum? We solve it by applying the principle of Uniformitarianism.
Most criticisms of Uniformitarianism start out with a fundamental misunderstanding of, or the intentional redefinition of, this basic geological principle.
UNIFORMITARIANISM VS CATASTROPHISM
Initial thinking on earth history was inspired by the bible. The recognition that major rock series are characterized by a distinct set of fossils lead to the belief that the fossils of each rock series were result of a creation and then were subsequently destroyed by some catastrophic event (e.g. the biblical flood). The main proponent of this theory was the French naturalist Georges Cuvier. In the 18th century there was even a case when some unfortunate geologist (Johann Jacob Scheuchzer, 1672-1733) found skeletons of giant salamanders and identified them as the victims of the biblical flood. The problem was that upon close inspection, these flood victims had long tails and sharp claws. Thus, it earned the proponent quite a bit of ridicule. Generally speaking, this way of looking at the geologic record, namely to assume that a series of immense, brief, and worldwide upheavals changed the earth greatly and produced mountains, valleys, and various other large scale features, came to be known as catastrophism.
The theory of catastrophism was challenged by James Hutton in the late 18th century, who in his theory of uniformitarianism proposed that uniform gradual processes (such as for example the slow erosion of the coast by the impact of waves) shaped the geologic record of the earth over an immensely long period of time. He assumed that the acting processes were the same than those that we see in action at present (rivers, volcanoes, waves, tides etc.). Darwin later on based his theory of the origin a species on Hutton’s theory.
The sedimentary structures that we saw earlier in this lecture serve as a good illustration how uniformitarianism works. Cross-bedding for example can be observed to form in modern river channels and also in experimental setups called flumes. We learn from these observations what kind of current velocities are needed to produce cross-bedding in a given grain size, and we realize that cross-bedding can be used as an indicator of current flow direction. We can apply what we learn from modern cross-beds to interpret the rock record in terms of flow velocities and flow direction. Likewise, finding ancient equivalents of modern mudcracks suggests to us that we look at sediments that dried out beneath the air, and were thus deposited on land.
In more modern times, some amendments have been made to the theory of uniformitarianism. One of these would be that it was recognized that catastrophic events are as much part of geologic history as the uniform action of the everyday processes. For example, sediment supply to the oceans is not a constant flux of matter. There is a considerable episodic component to sedimentation, e.g. storms are major agents of sediment redistribution in shelf seas, floods and exceptionally strong rains are responsible for most of the erosion and sediment redistribution on the continents. Undoubtedly, the physical and chemical principles (e.g. gravity, thermodynamics) that govern geologic processes of the present have also applied in the past. Yet as is visible in the present, frequent small deviations from equilibrium and unstable behavior (minor catastrophes, such as earthquakes, floods, storms) must have been an integral part of these processes. Similarly, the evolution of life was not a single succession of tiny evolutionary steps as originally envisioned by Darwin. We are now able to see that there were episodes of accelerated (punctuated) evolution, usually as a response to a change in environmental conditions, such as climate (ice ages, warming of the earth), the advent or immigration of new predators and the utilization of new food sources. Extremely rare (and catastrophic) events, such as the impact of large meteors, may have had a profound influence on our planet. Yet meteors fall onto the earth on a daily basis, just as it rains every day. In that sense, meteorite impacts are quite normal and part of the spectrum of everyday processes. Only very rarely does a “doomsday” meteorite that is 10 or more km in diameter hit the Earth and cause severe disruptions. To sum it up: The natural laws do not change with time and they have and will determine interior and external processes of the earth. Even the extremely rare event (e.g. meteor impacts) is part of the many geologic processes governed by these laws. Even though something, like for example the December 2004 tsunami, appears to us as a unique catastrophe, over the long run it is a normal and recurring event. It does not follow, however, that the rate of geologic processes is the same today as it was in the past. Some processes, such as mantle convection do probably stay stable over long time periods, but others, such as glaciation were at times very intense in the past (ice ages), but are presently less significant for continental erosion. So, a brief definition of Uniformitarianism would be: the natural laws that govern geologic processes have not changed over geologic time, but the rate at which certain geologic processes operate can vary. Uniformitarianism also has been paraphrased as “The Present is the Key to the Past“.
Uniformitarianism doesn’t preclude catastrophic events; nor does it stipulate that all processes must occur at a constant gradual rate. And it certainly doesn’t blind geologists to actual evidence of past catastrophic events, like impact features. Many of the world’s 190 confirmed impact craters (technically 189 because they count Upheaval Dome as confirmed) would be unknown if not for geologists employing uniformitarian methods to identify them. 34% of the confirmed impact craters are not exposed at the surface. 53% of the confirmed impact craters have been drilled, either intentionally or inadvertently while drilling for something else. The craters without surface expressions were identified by uniformitarian geologists/geophysicists interpreting geological and geophysical data.
Uniformitarianism says “The Present is the Key to the Past.” Understanding present day geological processes enables geologists to decipher the geologic past. It enables us to translate the language of the rocks.
So, what does this have to do with lunar recession?
Two words: Tidal Rhymites
The sedimentary record in Muir Inlet, a macrotidal fjord in Alaska, is dominated by cyclic silt–mud rhythmites. Couplet thicknesses vary systematically in a vertical sequence and reflect a semidiurnal tidal regime. Semimonthly, monthly, bimonthly, and annual cycles can be identified both visually in cores and by spectral frequency analysis. An average annual sedimentation of 22.5 cmyr-1 occurs over a four-month meltwater season and is confirmed by 210Pb dating. These modern deep-water tidal rhythmites can be used to verify interpretations made on ancient rhythmites in the stratigraphic record, and they also provide a dating tool to interpret high latitude successions for high-resolution climate change.
Cowan et al., 1998
Tidal rhymites are the result of very high frequency depositional cycles. The length of the day, the number of days in the month and months in the year can all be calculated from tidal rhythmites. Fortunately for geologists, Earth was kind enough to preserve at least a few very old tidal rhythmite deposits.
BIG COTTONWOOD CANYON
By Sandra Eldredge
This tour begins 1 billion years ago when the area was a tidal environment at an ocean shoreline. The tidal environment is preserved in the now-tilted layers of quartzite and shale that make up the canyon walls for the first 6 miles. In some areas, the shale is metamorphosed into argillite or slate.
One-billion-year-old records of the rhythm of ancient ocean tides
One of the best documented and oldest known records worldwide of tidal rhythmites is in Big Cottonwood Canyon. Discovered in the 1990s, this record is enthusiastically being researched, in large part to provide clues to ancient lunar cycles.
Yearly, monthly, and even daily and semi-daily tides are recorded in the black shale of the 850-million to 1-billion-year-old Big Cottonwood Formation.
Within the shale are thin, alternating layers of light-colored sand and dark-colored silt and clay. The sand was carried by peak (strong, dominant) flows and the silt and clay by slack (weaker, subordinate) waters at changing tides. Thus, these thin individual bands record daily tides and can be counted much like we count tree rings.
Because the gravitational pull of the moon and the sun cause tides, the length of an ancient day and lunar month can be determined from these tidal rhythmites. Long ago, the moon took less time to orbit the Earth, the Earth was spinning faster, and thus the days were shorter and there were more of them in a year. These records in stone indicate that one billion years ago, a day on Earth lasted only 18 hours, there were 13-plus months in a year, and about 481 days in a year!
(Information supplied by Marjorie A. Chan, University of Utah and Allen W. Archer, Kansas State University).
Utah Geological Survey
Tidal rhyhmites and other paleontological data tell us that the days have been getting longer, while the number of days per year have been decreasing over the past 500 million years.
Tidal rhymite formations enable geologists to reconstruct “the history of Earth’s rotation and lunar orbit” (Williams, 1990) over the past 2.5 billion years.
The recent recognition of cyclically laminated tidal rhythmites provides a new approach to tracing the dynamic history of the Earth-Moon system. Late Proterozoic (~650 Ma) elastic rhythmites in South Australia represent an unsurpassed palaeotidal record of ~560 years’ duration that provides numerous palaeorotational parameters. At~650 Ma there were 13.1 ±0.1 lunar months/year, 400 ±7 solar days/year, and 30.5 ±0.5 solar days/lunar month. The lunar apsides and lunar nodal cycles were then 9.7 ±0.1 years and 19.5 ±0.5 years, respectively. The indicated mean Earth-Moon distance of 58.28 ±0.30 Earth radii at ~650 Ma gives a mean rate of lunar retreat of 1.95 ±0.29 cm/year since that time, about half the present rate of lunar retreat of 3.7 ±0.2 cm/year obtained by lunar laser ranging. The rhythmite data imply a substantial obliquity of the ecliptic at ~650 Ma, and indicate virtually no overall change in the Earth’s moment of inertia, which militates against significant Earth expansion since ~650 Ma. Early Proterozoic ( ~2,500 Ma) cyclic banded iron-formation in Western Australia, that may record submarine fumarolic activity triggered by earth tides, suggests ~14.5 ±0.5 lunar months/year and a mean Earth-Moon distance of ~54.6 Earth radii at ~2,500 Ma. The combined rhythmite data suggest a mean rate of lunar retreat of ~1.27 cm/year during the Proterozoic (~2,500-650 Ma); the indicated increasing mean rate of lunar retreat since~2,500 Ma is consistent with increasing oceanic tidal dissipation as the Earth’s rotation slows. A close approach of the Moon during earlier time is uncertain. Continued study of tidal rhythmites promises to further illuminate the evolving dynamics of the Earth-Moon system.
The rate of lunar recession was highly variable during the Proterozoic Eon. The closest the closest approach of the Moon’s orbit to the Earth, as estimated from Precambrian tidal rhymites was ~51.9 Earth radii (~206,000 miles) approximately 2.45 billion years ago.
Green et al., 2017 indicate that the modern recession rate is anomalously high, about twice the average of the Proterozoic Eon. However, the rate appears the have even been higher than it currently is, during the Pleistocene Epoch’s Last Glacial Maximum.
It was recently shown through numerical tidal model simulations with higher resolution than in previous studies that the tidal dissipation during the early Eocene (50 Ma) was just under half of that at present (Green and Huber, 2013). This is in stark contrast to the Last Glacial Maximum (LGM, around 20 ka) when simulated tidal dissipation rates were significantly higher than at present due to changes in the resonant properties of the ocean (Green, 2010, Wilmes and Green, 2014, Schmittner et al., 2015). However, the surprisingly large tides during the LGM are due to a quite specific combination of continental scale bathymetry and low sea-level, in which the Atlantic is close to resonance when the continental shelf seas were exposed due to the formation of extensive continental ice sheets (Platzman et al., 1981, Egbert et al., 2004, Green, 2010). It is therefore reasonable to assume — and proxies support this — that the Earth has only experienced very large tides during the glacial cycles over the last 1–2 Ma and that the rates have been lower than at present during the Cenozoic (Palike and Shackleton, 2000, Lourens and Brumsack, 2001, Lourens et al., 2001). Such (generally) low tidal dissipation rates may have led to reduced levels of ocean mixing, with potential consequences for the large scale ocean circulation, including the Meridional Overturning Circulation (Munk, 1966, Wunsch and Ferrari, 2004).
Green et al., 2017
Science starts with observations (what we know) and then works to form hypotheses to explain the observations. The hypotheses are then tested to see if they can become scientific theories. Ideally, the hypotheses are tested empirically, in controlled experiments. Unfortunately, in geology, most hypothesis can only be tested by gathering more observations. This is why Chamberlin’s Method of Multiple Working Hypotheses is taught by many geology departments. For every given set of Earth Science observations, there are, almost invariably, multiple working hypotheses (non-uniqueness). As more observations are collected, some hypotheses will survive, others will have to be modified or discarded.
In the case of lunar recession, we started out with two observations:
- The Moon has been receding from the Earth at a rate of 3.8 cm/yr.
- The Earth and the Moon are approximately 4.5 billion years old.
We know the current recession rate from decades of laser ranging. While the ages of the Earth and the Moon aren’t known with as much certainty as the lunar recession rate, radiometric dating precludes them from being significantly younger than 4.5 billion years old.
At first glance, the physical cause of the first observation would seem to contradict the second observation. This means that either: 1) the recession rate was much slower in the past or 2) the Earth and Moon are less than 1.5 billion years old. Since we can’t run a controlled experiment, the best we can do is to see if the rocks can tell us anything about past recession rates. The rocks tell us that modern recession rate is anomalously high, and that the Moon’s orbit has not been catastrophically close to Earth at any point in the past 2.45 billion years. Without the Principle of Uniformitarianism, geologists wouldn’t be able to “translate the language” of the rocks.
Hat tip to hiskorr for his (or her) comments.
I had never given this subject any thought before. I actually found it quite interesting when I started digging into it.
Day 9 of America Held Hostage by CHICOM-19
Yesterday, Dallas County Commissioner Clay Jenkins issued a “shelter in place” order for Dallas County. Judge Jenkins, a liberal Democrat, looks like Howdy Doody and talks like Forrest Gump. The latest numbers indicate that there are 169 CHICOM-19 cases in Dallas County, and there have been 5 deaths. Dallas County has a population of
2,637,772 2,637,767 people.
|% of population||0.0064%||0.00019%|
99.9936% of Dallas County does not have CHICOM-19 and 99.9998% of us haven’t died from it.
The shelter in place order means that I get to work from home for at least another two weeks. So, I set up my work station on the island in our kitchen and have the TV on. The other day I watched The Big Lebowsky while I was working in the shorts and tee-shirt that I had slept in, wearing slippers. I could get used to this. Meetings are much more fun this way. Instead of a conference room, we get to WebEx and see our co-workers at home in shorts and flip-flops.
The shelter in place order only allows essential businesses to remain open, which, oddly, includes liquor stores (Hoo-Ahh!). As nearly as I can tell, most white collar workers have been told to work from home, if possible, while the businesses that mainly hire illegal aliens appear to be unaffected.
Cowan, E., Cai, J., Powell, R. et al. Modern tidal rhythmites deposited in a deep-water estuary. Geo-Marine Letters 18, 40–48 (1998). https://doi.org/10.1007/s003670050050
Green, Mattias & Huber, M. & Waltham, D. & Buzan, Jonathan & Wells, Martin. (2017). “Explicitly modelled deep-time tidal dissipation and its implication for Lunar history. ” Earth and Planetary Science Letters. 461. 46-53. 10.1016/j.epsl.2016.12.038.
Williams, G. E., “Tidal rhythmites: Key to the history of the Earth’s rotation and the lunar o”rbit”, J. Phys. Earth, 38, 475-491, 1990.
Williams, G.E. Geological Constraints on the Precambrian History of Earth’s Rotation and the Moon’s Orbit, Reviews of Geophysics 38 (2000), 37-59.
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